This invention relates to a new organic coating for glass surfaces, especially for non-returnable glass bottles and other glass containers, which protects the glass from scratching, thus increasing its breakage resistance, and also is capable of retaining glass fragments when a glass container is accidentally broken.
Accidental breakage of glass containers, especially bottles containing a carbonated or fermented liquid under gas pressure, can cause serious injury to persons and occasionally to property. Breakage is sometimes caused by imperfections in glass or by scratches and abrasion resulting from normal handling. Various types of coatings have been applied to glass containers to reduce the risk of breakage. It is customary in the glass container industry to apply to pristine containers molecular layers of titanium or tin oxides; often this is followed by a very thin organic coating, for example, of polyethylene or of polyoxyethylene stearate, at a temperature of about 93.degree.-204.degree. C. While these and similar forms of glass treatment have reduced the risk of breakage by strengthening the glass itself, they have not solved the problem of damage and injury in case of accidental breakage, especially in the case of pressurized containers.
Two approaches have been recently taken with respect to this particular problem. One, which applies to non-returnable containers, consists in coating the container with a thin layer of an organic, preferably polymeric material, which forms a film capable of retaining glass fragments after breakage. This coating must adhere to the glass sufficiently strongly to remain attached to it during normal filling and handling operations. Yet, the bond must not be so strong that the organic coating would not be able to delaminate on impact that causes breakage. It is this ability to delaminate that allows the coating to act as a protective shield because a major portion of impact energy is dissipated by viscoelastic deformation of the organic film, which is able to stretch without breaking.
The other approach, that applies to returnable bottles, also requires an organic coating, which in this case is considerably thicker and must be strongly bonded to glass. Such a coating must be able to withstand repeated alkaline washing, which of course is not required in the case of non-returnable glass containers.
Prior art has suggested the use of thermoplastic copolymers of .alpha.-olefins with .alpha.,.beta.-unsaturated short chain carboxylic acids as a particularly suitable material for coating both returnable and non-returnable glass containers. The basic patent covering the copolymers themselves is U.S. Pat. No. 3,264,272 to Rees. Various improvements in those coating processes have been proposed, including an invention directed to non-returnable glass containers described in U.S. Pat. No. 3,992,450 to Stengle. This patent provides a layer of polyethylene between the glass and the copolymer. The main advantage of the patented invention is said to reside in the compatibility of the coating with the primary protective organic coating applied to the pristine container at the factory.
While the process of U.S. Pat. No. 3,992,450 provides an excellent protective system, it has the drawback that the bond strength between the polyethylene coating and the glass decreases sharply at high humidity; for example, above 80% relative humidity (R.H.) bonding is very weak, and above 85% R.H. virtually no bonding remains. There thus is a need for a protective coating for non-returnable glass containers capable of absorbing the impact of breakage and of retaining the resulting cullet, which would operate satisfactorily within a broad range of humidities, including high R.H. levels.